Four Marines were killed Wednesday at Camp Pendleton in Southern California, while clearing the firing range of unexploded ordnance. A military investigation deemed that human error was the cause of the tragedy.

The accident occurred at 11 a.m. at the San Diego County coastal base, and Marines spokesman Lt. Ryan Finnegan said the clearance operation would have involved anything necessary to keep the range free of obstructions, which could include disposing of ordnance. The identities of the dead were withheld pending notification of relatives.

According to the military probe into the matter, an unnamed Marine operating a 60mm mortar tube and ammunition, did not follow proper procedures, which resulted in the detonation of a high explosive round at the mortar position. The probe also determined that the mortar team involved in the accident had not completed “appropriate preparatory training.”

A mortar is an indirect fire weapon that emits explosive projectiles known as mortar bombs, at low velocities, short ranges and high-arcing ballistic trajectories. It is typically muzzle-loading and has a barrel length less than 15 times its caliber.

Below is a clip of the 3rd United States Infantry Regiment (The Old Guard) loading, calibrating and firing mortar rounds:

Brigadier General John W. Bullard, commanding general of Marine Corps Installations West at Marine Corps Base Camp Pendleton, said in a statement, “We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident. Our first priority is to provide the families with the support they need during this difficult time.”

The Pendelton tragedy comes after 7 Marines were killed after an explosion during a live-fire training exercise at the Hawthorne Army Depot, roughly 100 miles south of Reno, Nevada.

Image via Wikimedia Commons.